1. Technical Field
The present invention relates to an image display medium in which colored particles are driven by electric fields for implementing repeatedly rewritable display, and to an image writing device that utilizes the image display medium.
2. Description of the Related Art
Heretofore, repeatedly rewritable image display mediums with excellent storage of a displayed image have been proposed. For example, a twisting ball display (display by rotation of particles that are portionally coated with two separate colors), magnetophoresis-type display mediums, thermal rewritable display mediums, liquid crystals having memory, and the like have been suggested thereas.
However, these image display mediums have problems in that they are unable to display white as ordinary paper does and image contrast is low.
Image display mediums that utilize two kinds of colored particles having different colors have been proposed as image display mediums for solving the problem mentioned above. For example, an image display medium described in Japan Hardcopy '99 Papers, pp. 249-252 and Japan Hardcopy '99 Fall Proceedings, pp. 10-13, is structured with a transparent display substrate, a back substrate which opposes the display substrate with a very small gap therebetween, and electro-conductive black toner and insulative white toner enclosed between the substrates. Electrodes are formed at the display substrate and the rear substrate, and an inner face of each electrode is coated with a charge transport material which transports charge carriers of only one polarity (for example, positive holes). When an electric field is applied between these substrates, the positive holes are injected only to the electro-conductive black toner, and the black toner is positively charged. Thus, the black toner pushes between the white particles in response to the electric field formed between the substrates, and moves between the substrates. Black display is implemented when the black toner is moved to the display substrate side, and white display is implemented by the white particles when the black toner is moved to the back substrate side.
A further image display medium described in Japanese Patent Application Laid-Open No. 2001-33833 is structured with opposing substrates provided with an electro-conductive layer or a commutative layer, at least a display face side being transparent, and two kinds of colored particles, which have mutually different colors and are not affected by charges of the same polarity, enclosed between the substrates. In this image display medium, charge transfer from the electro-conductive layers or commutative layers to the colored particles is implemented, colored particles to which charge has been donated are moved by operation of an electric field, and an arbitrary kind of the colored particles is adhered to the display substrate. Thus, image display is implemented.
A still further image display medium described in Japanese Patent Application Laid-Open No. 2001-312225 is structured with a transparent display substrate, a back substrate which opposes the display substrate with a very small gap therebetween, and two kinds of particle groups, which have different colors and charge polarities, enclosed between the substrates. The two kinds of particle groups are charged with mutually opposite polarities. When voltage is applied between the substrates, the particle groups move to different substrate sides from one another. Image display is implemented by applying voltage between the substrates in accordance with image information, and adhering arbitrary kinds of the colored particles to the display substrate.
A yet further image display medium described in Japanese Patent Application Laid-Open No. 2001-242492 is structured with a pair of substrates, at least a display surface side of which is transparent, a highly insulative uncolored dispersion medium enclosed between the substrates, and at least two kinds of electrophoretic fine particles, which have mutually different electrophoresis characteristics, enclosed between the substrates. The two different kinds of electrophoretic fine particles are composed of on the one hand white particles and on the other hand colored particles having a different color tone from the white particles. The two kinds of particles are charged to mutually opposite polarities. When voltage is applied between the substrates, the two kinds of particles move to different substrate sides from one another. Image display is implemented by applying voltage between the substrates in accordance with image information, and adhering arbitrary kinds of the colored particles to the display substrate.
In these image display mediums, images are displayed by adhering either one of two kinds of colored particles at a transparent display substrate face in accordance with image information. By using colored particles with high light suppression characteristics, image display with high clarity of contrast can be implemented by the two kinds of particles having different colors.
In order to implement multi-color display with an image displaying medium that employs two kinds of colored particles having different colors, first, there is a type of display for displaying multi-color images in which color filters are formed at a display substrate. When a color filter has been formed at a display substrate, a mixed color of a color of particles that are adhered at an inner face of the display substrate and the color of the color filter is displayed. For example, if white particles and black particles are employed, then the color of the color filter is displayed when the white particles are adhered at the display substrate, and black display is implemented when the black particles are adhered thereat. As such color filters, for example, R, G and B filters may be employed, and a single color pixel can be structured by three or more neighboring pixels of red, green, and blue. The black and white particles are moved in accordance with image signals, and reflection of R, G and B light is controlled. Thus, arbitrary colors can be displayed.
A method in which a cell structure is divided up by a spacer between a display substrate and a back substrate, colored particles having different colors are enclosed in the respective cells, and a single color is expressed by a number of neighboring cells has been offered as another method. For example, there is a case in which three kinds of particle combinations, that is, black particles and red particles, black particles and green particles, and black particles and blue particles, are enclosed in respective cells in a regular manner, and a single color pixel is expressed by three neighboring cells of red, green and blue. The black particles and the red, green and blue particles are moved in accordance with image signals, and reflection of R, G and B light can be controlled to display freely selected colors.
However, because in these methods a single color pixel is expressed by a plurality of pixels which control light reflection of each of red, green and blue, resolution of a displayed image is reduced. In particular, a deterioration of text quality is remarkable. To maintain resolution for text display, it is necessary to produce display mediums of higher resolution, but higher costs for driving circuits and increases in production costs are unavoidable.
Furthermore, in the multi-color display methods described above, white display is affected by a grayish tone, and whiteness of the display, which is a major feature for image display mediums that employ colored particles, is lost. Moreover, because a color is displayed by the color of a selection of red, green and blue juxtaposed in a color pixel, the multi-color display is dark overall. This is acceptable in an environment in which the surroundings are bright, but in an environment in which the surroundings are dark, there is a problem in that a deterioration of display quality of the multi-color display image is remarkable.
In the multi-color display methods described above, a deterioration in text quality due to a reduction of resolution, a reduction in white display quality or black display quality, a reduction in display quality of multi-color images in an environment in which the surroundings are dark, and the like are fundamental problems that cannot be ameliorated even if the color structure of color filters, the color combinations of the enclosed colored particles and the like are changed.